xen-netfront: use netif_start_queue() on xennet_open()

xen-netfront never called netif_start_queue() and was was waking the
queue on xennet_open(), triggering the BUG_ON() on __netif_schedule().
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/net/xen-netfront.c

@@ -329,7 +329,7 @@ static int xennet_open(struct net_device *dev)
 	}
 	spin_unlock_bh(&np->rx_lock);
 
-	xennet_maybe_wake_tx(dev);
+	netif_start_queue(dev);
 
 	return 0;
 }